PURPOSE: In rowing, the effectiveness of adding high-intensity interval training (HIIT) to moderate-intensity continuous training (MICT) within the weekly training program on physiological adaptations and performance is still unclear. This study compared the effects of HIIT plus MICT (MIXED) versus MICT alone on physiological/metabolic responses and performance in adolescents. METHODS: Twelve highly trained adolescent rowers (age: 15.7 [0.5] y) were divided into 2 groups: MIXED and MICT. Before and after a 7-week intervention period, rowers underwent an incremental step test to determine peak oxygen uptake (VO2peak), power at VO2peak (WVO2peak), power corresponding to a lactate concentration of 2 and 4 mmol·L-1, power output at lactate threshold, oxygen uptake at the second lactate threshold (VO2LT), and peak oxygen pulse. Training load from TRIMP was also measured. The training intervention consisted of 7 sessions per week including 2 "off-water," 3 "on-water," and 2 resistance-training sessions. The "on-water" and resistance-training sessions were the same for both groups, while during "off-water" sessions, the MIXED group performed HIIT (4 × 4 min at 85% WVO2peak) and the MICT group performed moderate-intensity training (80 min at 70% WVO2peak). RESULTS: Statistical analysis showed that in the MIXED group, VO2LT was significantly increased and training load from TRIMP was significantly reduced (P <
.00001) compared with the MICT group (P = .008). Both groups similarly improved VO2peak, peak oxygen pulse, WVO2peak, power output at lactate threshold, and power corresponding to a lactate concentration of 2 and 4 mmol·L-1. CONCLUSIONS: Our findings showed that, in adolescent rowers, MIXED training enhanced VO2LT, thus indicating HIIT as a valid and time-efficient addition to traditional MICT. However, given that adolescents were examined, data should be interpreted with caution, as training and/or growth/maturation may have contributed to performance changes.